The present invention relates to urea-formaldehyde resins, including melamine-urea-formaldehyde resins, and in particular to compositions for enhancing the polymerization of such resins.
Urea-formaldehyde (UF) resins have been used for over forty years as binders for reconstituted wood-based panel products, for example interior grade particleboard, medium density fiberboard (MDF), and plywood. In use, the UF resin is combined with “furnish material”, i.e., particles, flakes, fibers, or sheets of a cellulosic or lignocellulosic material, formed into mats where appropriate, stacked, and compressed between heated platens in a hot press to form a board product of the desired thickness and density. These board products are often subjected to secondary hot pressing operations, including lamination. UF resins are used because they result in boards with high physical strength properties, allow board production at relatively high production rates and minimal energy consumption, and are much more cost-effective than alternative binders.
UF resins are commonly cured using a combination of a moderate increase in acidity and elevated temperatures. The increase in acidity may be brought about by contact with the wood product and/or by the addition of an acid-generating catalyst, typically the ammonium salts of sulfuric, hydrochloric or nitric acids. Such catalysts allow faster cure in the presence of lower amounts of formaldehyde; however, their catalytic efficiency decreases significantly at F:U mole ratios less than 1.0. After resin cure, the continued presence of the acid catalysts can catalyze the reverse reaction, causing polymer degradation especially during hot stacking of the board and/or during secondary hot-pressing operations. To significantly mitigate this latter effect, a buffered catalyst system has been developed, as disclosed in U.S. Pat. No. 5,635,583 to Motter et al. This catalyst system comprises an aqueous solution of a buffering salt (preferably dipotassium phosphate) adjusted to a pH of about 8.5 to about 2.0, phosphoric acid, and ammonium sulfate. The buffering salt solution most preferably includes an acid to achieve a pH in the lower half or below the buffering range of the buffering salt solution.
UF resins have also been modified by the incorporation of low amounts (0.1 to 5.0 weight %) of melamine to produce low F:U mole ratio (typically <1.0:1.0) hybrid (mUF) resins that resist degradation of polymer properties during hot stacking and secondary pressing operations, and that have low formaldehyde emissions. Overall, these resins cost more than conventional UF resins, require significantly higher application rates to achieve adequate physical board properties, and require significantly more cure time (and/or higher energy input). Conventional ammonium salt-based catalyst systems are not as effective in these mUF systems.
Additionally, a class of higher melamine content (>5 to about 30%) UF resins (MUF) have been used to manufacture products for applications requiring enhanced resistance to moisture. The cost, required rate of application and time required for cure of these resins generally increases in proportion to the melamine content.
There accordingly remains a need in the art for methods to enhance the polymerization of UF, mUF and/or MUF resins. In particular, there remains a need for compositions and methods that will improve the cure speed and/or lower the cure temperature for UF, mUF and/or MUF resins. Preferably such compositions and methods not significantly adversely impact, or will even improve, the advantageous properties of the resins.